Photoelectric conversion device and imaging system

ABSTRACT

A photoelectric conversion device includes a plurality of pixels each of which includes a photoelectric converter that generates charges by photoelectric conversion, a first transfer unit that transfers charges in the photoelectric converter to a first holding portion, a second transfer unit that transfers charges in the first holding portion to a second holding portion, an amplifier unit that outputs a signal based on charges held in the second holding portion, and a third transfer unit that transfers charges of the photoelectric converter to a drain portion; and a control unit that, in an exposure period in which signal charges are accumulated in the photoelectric converter, changes a potential barrier formed by the third transfer unit with respect to the signal charges accumulated in the photoelectric converter from a first level to a second level that is higher than the first level.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a photoelectric conversion device andan imaging system.

Description of the Related Art

In a photoelectric conversion device in which a plurality of pixels forman imaging surface, so-called global electronic shutter is known inwhich the start time and the end time of charge accumulation operationsare the same in all the pixels within the imaging surface. JapanesePatent Application Laid-Open No. 2013-172204 discloses a photoelectricconversion device having a global electronic shutter function.

Each pixel in the photoelectric conversion device disclosed in JapanesePatent Application Laid-Open No. 2013-172204 includes a photoelectricconverter that generates charges, a first transfer switch that transferscharges from the photoelectric converter to a first holding portion, anda second transfer switch that transfers charges from the first holdingportion to an FD region. Further, the pixel in the photoelectricconversion device disclosed in Japanese Patent Application Laid-Open No.2013-172204 includes a third transfer switch that drains charges fromthe photoelectric converter to an overflow drain. With the same timingthat stops draining charges from the photoelectric converter to theoverflow drain in all the pixels, it is possible to have the same starttime of the exposure period for all the pixels. Further, with the sametiming of transferring charges from the photoelectric converter to thefirst holding portion in all the pixels, it is possible to have the sameend time of the exposure period for all the pixels. This allows a globalelectronic shutter operation to be realized.

Japanese Patent Application Laid-Open No. 2013-172204 discloses that,during the exposure period of the photoelectric converter, the potentialbarrier of the third transfer switch with respect to charges accumulatedin the photoelectric converter is set lower than the potential barrierof the first transfer switch with respect to charges accumulated in thephotoelectric converter. This can prevent charges generated during anexposure period of the photoelectric converter from overflowing to thefirst holding portion side.

With a lower potential barrier of the third transfer switch during theexposure period of the photoelectric converter, however, the amount ofsignal charges that can be accumulated in the photoelectric converter(saturation charge amount) decreases resulting in a narrower dynamicrange of the output signal. On the other hand, with a higher potentialbarrier of the third transfer switch during the exposure period of thephotoelectric converter for increasing the saturation charge amount ofthe photoelectric converter, leakage of charges into the first holdingportion from the photoelectric converter may occur, which may besuperimposed as noise on the previous frame signal.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a photoelectricconversion device and a method of driving the same that can preventoccurrence of a false signal due to leakage of charges into a holdingportion from a photoelectric converter while suppressing reduction inthe saturation charge amount of a photoelectric converter.

According to an aspect of the present invention, there is provided aphotoelectric conversion device including a plurality of pixels each ofwhich includes a photoelectric converter that generates charges byphotoelectric conversion, a first transfer unit that transfers chargesin the photoelectric converter to a first holding portion, a secondtransfer unit that transfers charges in the first holding portion to asecond holding portion, an amplifier unit that outputs a signal based oncharges held in the second holding portion, and a third transfer unitthat transfers charges of the photoelectric converter to a drainportion, and a control unit that, in an exposure period in which signalcharges are accumulated in the photoelectric converter, changes apotential barrier formed by the third transfer unit with respect to thesignal charges accumulated in the photoelectric converter from a firstlevel to a second level that is higher than the first level.

Further, according to another aspect of the present invention, there isprovided a photoelectric conversion device including a plurality ofpixels each of which includes a photoelectric converter that generatescharges by photoelectric conversion, a first transfer unit thattransfers charges in the photoelectric converter to a first holdingportion, a second transfer unit that transfers charges in the firstholding portion to a second holding portion, an amplifier unit thatoutputs a signal based on charges held in the second holding portion,and a third transfer unit that transfers charges of the photoelectricconverter to a drain portion, an amplifier circuit that amplifies asignal based on signal charges by a gain that can be selected from atleast a first gain and a second gain different from the first gain, anda control unit that controls a potential barrier formed by the thirdtransfer unit with respect to the signal charges accumulated in thephotoelectric converter to a first level when the gain is the first gainand controls a potential barrier of the third transfer unit with respectto the signal charges to a second level different from the first levelwhen the gain is the second gain.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a general configuration of aphotoelectric conversion device according to a first embodiment.

FIG. 2 is a circuit diagram illustrating a configuration example ofpixels of the photoelectric conversion device according to the firstembodiment.

FIG. 3 is a schematic cross-sectional view illustrating the structure ofa pixel of the photoelectric conversion device according to the firstembodiment.

FIG. 4 and FIG. 6 are timing diagrams illustrating drive timings ofrespective control signals in a typical global electronic shutteroperation of the photoelectric conversion device.

FIG. 5A, FIG. 5B, FIG. 5C, FIG. 5D, FIG. 5E and FIG. 5F are diagramsschematically illustrating changes in potential states of respectiveportions of the pixel.

FIG. 7 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a first comparativeexample.

FIG. 8 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a second comparativeexample.

FIG. 9A is a diagram schematically illustrating a change in potentialstates of respective portions of a pixel in the first comparativeexample and the second comparative example.

FIG. 9B is a diagram schematically illustrating a change in potentialstates of respective portions of a pixel in the second comparativeexample.

FIG. 10 is a timing diagram illustrating a method of driving thephotoelectric conversion device according to the first embodiment.

FIG. 11 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a third embodiment.

FIG. 12 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a third comparativeexample.

FIG. 13 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a fourth embodiment.

FIG. 14 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a fifth embodiment.

FIG. 15 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a fourth comparativeexample.

FIG. 16 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a sixth embodiment.

FIG. 17 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a seventh embodiment.

FIG. 18 is a timing diagram illustrating an object of the method ofdriving the photoelectric conversion device according to the fourthembodiment.

FIG. 19 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to an eighth embodiment.

FIG. 20 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a ninth embodiment.

FIG. 21A and FIG. 21B are diagrams illustrating a method of driving aphotoelectric conversion device according to a tenth embodiment.

FIG. 22 is a circuit diagram illustrating a configuration example of apixel of a photoelectric conversion device according to an eleventhembodiment.

FIG. 23 is a block diagram illustrating a general configuration of animaging system according to a twelfth embodiment.

FIG. 24A is a diagram illustrating a configuration example of an imagingsystem according to a thirteenth embodiment.

FIG. 24B is a diagram illustrating a configuration example of a movableobject according to the thirteenth embodiment.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail in accordance with the accompanying drawings.

[First Embodiment]

A photoelectric conversion device and a method of driving the sameaccording to a first embodiment of the present invention will bedescribed with reference to FIG. 1 to FIG. 10.

First, the configuration of the photoelectric conversion deviceaccording to the present embodiment will be described by using FIG. 1 toFIG. 3 with an example of a CMOS image sensor. FIG. 1 is a block diagramillustrating a general configuration of the photoelectric conversiondevice according to the present embodiment. FIG. 2 is a circuit diagramillustrating a configuration example of pixels of the photoelectricconversion device according to the present embodiment. FIG. 3 is aschematic cross-sectional view illustrating the structure of a pixel ofthe photoelectric conversion device according to the present embodiment.

As illustrated in FIG. 1, a photoelectric conversion device 100according to the present embodiment includes an imaging region 10, avertical scanning circuit 20, a readout circuit 30, a horizontalscanning circuit 40, an output circuit 50, and a control circuit 60. Thephotoelectric conversion device 100 can be formed of a single chip usinga semiconductor substrate.

In the imaging region 10, a plurality of pixels 12 arranged in a matrixover a plurality of rows and a plurality of columns are provided. Eachof the pixels 12 includes a photoelectric conversion element thatconverts an incident light into charges in accordance with the lightamount thereof. The number of rows and the number of columns of a pixelarray arranged in the imaging region 10 are not limited in particular.Further, in the imaging region 10, other pixels (not illustrated) suchas an optical black pixel that is shielded from light, a dummy pixelthat outputs no signal, or the like may be arranged in addition to thepixels 12 that output signals in accordance with the light amount of anincident light.

A control signal line 14 is arranged on each row of the pixel array ofthe imaging region 10 extending in the row direction (horizontaldirection in FIG. 1). The control signal line 14 is connected to thepixels 12 aligned in the row direction to form a signal line common tothese pixels 12. Further, a vertical output line 16 is arranged on eachcolumn of the pixel array of the imaging region 10 extending in thecolumn direction (vertical direction in FIG. 1). The vertical outputline 16 is connected to the pixels 12 aligned in the column direction,respectively, to form a signal line common to these pixels 12.

The control signal lines 14 on respective rows are connected to thevertical scanning circuit 20. The vertical scanning circuit 20 is acontrol circuit that supplies, to the pixels 12 via the control signallines 14 provided on a row basis of the pixel array, control signals fordriving the readout circuit 30 within the pixels 12 when reading outsignals from respective pixels 12. The vertical scanning circuit 20 canbe configured using a shift resistor or an address decoder. Signals readout from the pixels 12 are input to the readout circuit 30 via thevertical output lines 16 provided on a column basis of the pixel array.

The readout circuit 30 is a circuit unit that performs a predeterminedprocess, for example, signal processing such as an amplificationprocess, an addition process, or the like on the signals read out fromthe pixels 12. The readout circuit 30 may include signal holding units,column amplifiers, correlated double sampling (CDS) circuits, addercircuits, or the like. The readout circuit 30 may further include ananalog-to-digital (A/D) converter circuit or the like if necessary.

The horizontal scanning circuit 40 is a circuit unit that supplies, tothe readout circuit 30, control signals used for transferring signalsprocessed in the readout circuit 30 to the output circuit 50sequentially on a column basis. The horizontal scanning circuit 40 canbe configured using a shift resistor or an address decoder. The outputcircuit 50 is a circuit unit that is formed of a buffer amplifier, adifferential amplifier, or the like to amplify and output a signal on acolumn selected by the horizontal scanning circuit 40.

The control circuit 60 is a circuit unit that supplies, to the verticalscanning circuit 20, the readout circuit 30, and the horizontal scanningcircuit 40, control signals for controlling the operation or the timingthereof. Some or all of the control signals supplied to the verticalscanning circuit 20, the readout circuit 30, and the horizontal scanningcircuit 40 may be supplied from the outside of the photoelectricconversion device 100.

FIG. 2 is a circuit diagram illustrating an example of pixel circuitsforming the imaging region 10. While FIG. 2 depicts nine pixels 12arranged in three rows by three columns out of the pixels 12 forming theimaging region 10, the number of pixels 12 forming the imaging region 10is not limited in particular.

Each of the plurality of pixels 12 includes a photoelectric converterPD, transfer transistors M1, M2, and M6, a reset transistor M3, anamplifier transistor M4, and a select transistor M5. The photoelectricconverter PD is a photodiode, for example. The anode of the photodiodeof the photoelectric converter PD is connected to a ground voltage line,and the cathode is connected to the source of the transfer transistor M1and the source of the transfer transistor M6. The transfer transistor M6may be referred to as an overflow transistor, a charge drain transistor,or the like. The drain of the transfer transistor M1 is connected to thesource of the transfer transistor M2. The connection node of the drainof the transfer transistor M1 and the source of the transfer transistorM2 includes a capacitance component and has a function of a holdingportion of charges. In FIG. 2, this capacitance component is representedas a capacitor (C1), one terminal of which is connected to the node. Inthe following description, this capacitor may be denoted as a holdingportion C1. The other terminal of the capacitor forming the holdingportion C1 is grounded.

The drain of the transfer transistor M2 is connected to the source ofthe reset transistor M3 and the gate of the amplifier transistor M4. Theconnection node of the drain of the transfer transistor M2, the sourceof the reset transistor M3, and the gate of the amplifier transistor M4is a so-called floating diffusion (FD) portion. The FD portion includesa capacitance component (floating diffusion capacitor) and has afunction of a holding portion of charges. In FIG. 2, this capacitancecomponent is represented as a capacitor (C2), one terminal of which isconnected to the FD portion. In the following description, the FDportion may be denoted as a holding portion C2. The other terminal ofthe capacitor forming the holding portion C2 is grounded.

The drain of the reset transistor M3 and the drain of the amplifiertransistor M4 are connected to a power source voltage line (VDD).Further, the drain of the transfer transistor M6 is connected to a powersource voltage line (VOFD) that functions as an overflow drain OFD. Notethat any two or three of a voltage supplied to the drain of the resettransistor M3, a voltage supplied to the drain of the amplifiertransistor M4, and a voltage supplied to the drain of the transfertransistor M6 may be the same, or all of the above may be different. Thesource of the amplifier transistor M4 is connected to the drain of theselect transistor M5. The source of the select transistor M5 isconnected to the vertical output line 16.

In the case of the pixel configuration of FIG. 2, each of the controlsignal lines 14 arranged in the imaging region 10 includes signal linesTX1, TX2, TX3, RES, and SEL. The signal line TX1 is connected to thegates of the transfer transistors M1 of the pixels 12 belonging to thecorresponding row, respectively, and forms a signal line common to thesepixels 12. The signal line TX2 is connected to the gates of the transfertransistors M2 of the pixels 12 belonging to the corresponding row,respectively, and forms a signal line common to these pixels 12. Thesignal line TX3 is connected to the gates of the transfer transistors M6of the pixels 12 belonging to the corresponding row, respectively, andforms a signal line common to these pixels 12. The signal line RES isconnected to the gates of the reset transistors M3 of the pixels 12belonging to the corresponding row, respectively, and forms a signalline common to these pixels 12. The signal line SEL is connected to thegates of the select transistors M5 of the pixels 12 belonging to thecorresponding row, respectively, and forms a signal line common to thesepixels 12. Note that, in FIG. 2, the corresponding row number isprovided to the name of each control line (for example, TX1(n),TX1(n+1), TX1(n+2)).

A control signal PTX1 that is a drive pulse for controlling the transfertransistor M1 is output to the signal line TX1 from the verticalscanning circuit 20. A control signal PTX2 that is a drive pulse forcontrolling the transfer transistor M2 is output to the signal line TX2from the vertical scanning circuit 20. A control signal PTX3 that is adrive pulse for controlling the transfer transistor M6 is output to thesignal line TX3 from the vertical scanning circuit 20. A control signalPRES that is a drive pulse for controlling the reset transistor M3 isoutput to the signal line RES from the vertical scanning circuit 20. Acontrol signal PSEL that is a drive pulse for controlling the selecttransistor M5 is output to the signal line SEL from the verticalscanning circuit 20. When each transistor is formed of an n-channeltransistor, the corresponding transistor is turned on when supplied witha high level (hereafter, referred to as “Hi-level”) control signal fromthe vertical scanning circuit 20. Further, the corresponding transistoris turned off when supplied with a low level (hereafter, referred to as“Lo-level”) control signal from the vertical scanning circuit 20.

The vertical output line 16 arranged on each column of the imagingregion 10 is connected to the sources of the select transistors M5 ofthe pixels 12 aligned in the column direction, respectively, and forms asignal line common to these pixels 12. Note that the select transistorM5 of the pixel 12 may be omitted. In this case, the vertical outputline 16 is connected to the sources of the amplifier transistors M4.

The photoelectric converter PD converts (photoelectrically converts) anincident light into charges in accordance with the light amount thereofand accumulates the generated charges. The transfer transistor M6 resetsthe photoelectric converter PD to a predetermined electrical potentialin accordance with the voltage of the power source voltage line VOFD. Inother words, the transfer transistor M6 is a transfer unit thattransfers charges held by the photoelectric converter PD to the powersource voltage line VOFD. The transfer transistor M1 is a transfer unitthat transfers charges held in the photoelectric converter PD to theholding portion C1. The holding portion C1 holds charges generated bythe photoelectric converter PD in a different location from thephotoelectric converter PD. The transfer transistor M2 is a transferunit that transfers charges held in the holding portion C1 to theholding portion C2. The holding portion C2 holds charges transferredfrom the holding portion C1 and sets the voltage of the FD portion,which is also the input node of an amplifier unit (the gate of theamplifier transistor M4), to a voltage in accordance with thecapacitance value of the holding portion C2 and the amount of thetransferred charges. The reset transistor M3 is a reset unit that resetsthe holding portion C2 to a predetermined electrical potential inaccordance with the voltage of the power source voltage line VDD. Theselect transistor M5 selects the pixels 12 which output signals to thevertical output lines 16. In the amplifier transistor M4, the drain issupplied with the power source voltage, and the source is supplied witha bias current from a current source via the select transistor M5, whichforms the amplifier unit (source follower circuit) whose gate is theinput node. Thereby, the amplifier transistor M4 outputs a signal VOUTbased on charges generated by an incident light to the vertical outputline 16. Note that, in FIG. 2, the corresponding column number isprovided to the signal VOUT (Vout(m), Vout(m+1), Vout(m+2)).

FIG. 3 is a partial cross-sectional view of the pixel 12 formed on asemiconductor substrate. FIG. 3 depicts the transfer transistors M1, M2,and M6, the photoelectric converter PD, the holding portions C1 and C2,and an overflow drain OFD out of the components of the pixel 12. Theoverflow drain OFD is the drain of the transfer transistor M6 andfunctions as a drain portion of charges from the photoelectric converterPD. In this example, the conductivity type of each semiconductor regionwill be described with an example where electrons are used as signalcharges. When holes are used as signal charges, the conductivity type ofeach semiconductor region is the opposite conductivity type.

A p-type semiconductor region 112 forming a well is provided in thesurface portion of the n-type semiconductor substrate 110. Note that thesemiconductor substrate 110 may be of a p-type and, in this case, thesemiconductor substrate 110 itself may be the p-type semiconductorregion 112. Element isolation regions 114 defining an active region areprovided on the surface of the p-type semiconductor region 112. Theelement isolation region 114 is an insulating region formed by STI(Shallow Trench Isolation) method or LOCOS (LOCal Oxidation of Silicon)method, for example.

The overflow drain OFD, the transfer transistor M6, the photoelectricconverter PD, the transfer transistor M1, the holding portion C1, thetransfer transistor M2, and the holding portion C2 are arrangedneighbored in this order in the active region defined by the elementisolation region 114. Note that the reset transistor M3, the amplifiertransistor M4, and the select transistor M5, which are other componentsof the pixel 12, are provided in another active region (notillustrated).

The photoelectric converter PD includes an n-type semiconductor region116 forming a p-n junction to the p-type semiconductor region 112. Then-type semiconductor region 116 has the same polarity as electrons thatare signal charges and serves as a charge accumulation layer thataccumulates signal charges generated by the photoelectric converter PD.A p-type semiconductor region 118 is provided in the surface portion ofthe p-type semiconductor region 112 over the n-type semiconductor region116. The p-type semiconductor region 118 is used for forming thephotoelectric converter PD as a so-called buried photodiode structureand serves to suppress an influence of a dark current caused by aninfluence of an interface state of the surface of the semiconductorsubstrate 110.

The holding portion C1 includes an n-type semiconductor region 120provided spaced apart from the n-type semiconductor region 116 on thesurface of the p-type semiconductor region 112. The n-type semiconductorregion 120 serves as a charge holding layer that holds signal chargestransferred from the photoelectric converter PD. The holding portion C1may be the buried diode structure similar to the photoelectric converterPD.

A gate electrode 124 is provided over the semiconductor substrate 110between the n-type semiconductor region 116 and the n-type semiconductorregion 120 with a gate insulating film 122 interposed therebetween.Thereby, the transfer transistor M1 is formed in which the gateelectrode 124 is the gate, the n-type semiconductor region 116 is thesource, and the n-type semiconductor region 120 is the drain. Thepotential state between the photoelectric converter PD and the holdingportion C1 can be controlled by a voltage supplied to the gate electrode124. For example, it is possible for the transfer transistor M1 to forma potential barrier with respect to signal charges accumulated in then-type semiconductor region 116. Under the gate electrode 124, an n-typesemiconductor region 126 having a lower impurity concentration than then-type semiconductor region 116 is provided within the p-typesemiconductor region 112 between the n-type semiconductor region 116 andthe n-type semiconductor region 120.

The holding portion C2 includes an n-type semiconductor region 128provided spaced apart from the n-type semiconductor region 120 in thesurface portion of the p-type semiconductor region 112. The n-typesemiconductor region 128 serves as a charge holding layer that holdssignal charges transferred from the holding portion C1. On the n-typesemiconductor region 128, a contact plug 130 for electrically connectingthe holding portion C1 to the source of the reset transistor M3 and thegate of the amplifier transistor M4 (not illustrated) is provided.

A gate electrode 134 is provided over the semiconductor substrate 110between the n-type semiconductor region 120 and the n-type semiconductorregion 128 with a gate insulating film 132 interposed therebetween.Thereby, the transfer transistor M2 is formed in which the gateelectrode 134 is the gate, the n-type semiconductor region 120 is thesource, and the n-type semiconductor region 128 is the drain. Thepotential state between the holding portion C1 and the holding portionC2 can be controlled by a voltage supplied to the gate electrode 134.

The overflow drain OFD includes an n-type semiconductor region 136provided spaced apart from the n-type semiconductor region 116 in thesurface portion of the p-type semiconductor region 112. On the n-typesemiconductor region 128, a contact plug 138 for electrically connectingthe n-type semiconductor region 136 to the power source voltage lineVOFD (not illustrated) is provided.

A gate electrode 142 is provided over the semiconductor substrate 110between the n-type semiconductor region 116 and the n-type semiconductorregion 136 with a gate insulating film 140 interposed therebetween.Thereby, the transfer transistor M6 is formed in which the gateelectrode 142 is the gate, the n-type semiconductor region 116 is thesource, and the n-type semiconductor region 136 is the drain. Thepotential state between the photoelectric converter PD and the overflowdrain OFD can be controlled by a voltage supplied to the gate electrode142. For example, it is possible for the transfer transistor M1 to forma potential barrier with respect to signal charges accumulated in then-type semiconductor region 116.

Over the semiconductor substrate 110, a light-shielding film 144 forpreventing an incident light to the imaging region 10 from reaching theholding portion C1 is provided. To this end, the light-shielding film144 is provided so as to cover at least the holding portion C1. In termsof a higher light-shielding performance, it is preferable to arrange thelight-shielding film 144 so as to extend from the holding portion C1 andcover the entire gate electrode 124 and a part of the gate electrode 134as illustrated in FIG. 3, for example.

Next, a basic global electronic shutter operation in a photoelectricconversion device will be described using FIG. 4 to FIG. 5F. FIG. 4 is atiming diagram illustrating the drive timing of each control signal in aglobal electronic shutter operation. FIG. 5A to FIG. 5F are diagramsschematically illustrating potential states of respective portions ofthe pixel 12 at respective timings. FIG. 5A to FIG. 5F are diagramsillustrating the potential to an electron, and the potential toelectrons is higher, that is, the electrical potential is lower in theupper side in FIG. 5A to FIG. 5F. FIG. 5A to FIG. 5F illustrates theelectrical potentials of the overflow drain OFD, the transfer transistorM6, the photoelectric converter PD, the transfer transistor M1, theholding portion C1, the transfer transistor M2, and the holding portionC2.

In FIG. 4, the period before the time t1 is an exposure period of thek-th frame. In the period before the time t1, the control signals PTX1and PTX3 are at the Lo-level, that is, the transfer transistors M1 andM6 are in an off-state, and the potential to electrons in the transfertransistors M1 and M6 part is higher. Thereby, signal charges inaccordance with a light amount of an incident light is generated in thephotoelectric converter PD, and the generated signal charges areaccumulated in the photoelectric converter PD. FIG. 5A illustrates thepotential states of respective portions immediately before the time t1.

Note that, in the period before the time t1, the control signal PRES isat the Hi-level, that is, the reset transistor M3 is in an on-state, andthe holding portions C2 (FD portion) of all the pixels 12 are in a resetstate. Further, the control signal PSEL is at the Lo-level, that is, theselect transistor M5 is off-state, and all the pixels 12 are anot-selected state.

At the time t1, the control signals PTX1 on all the rows are controlledfrom the Lo-level to the Hi-level by the vertical scanning circuit 20,and the transfer transistors M1 of the pixels 12 on all the rows areturned on. Thereby, the potential barrier with respect to electrons ofthe transfer transistor M1 part decreases, and signal chargesaccumulated in the photoelectric converter PD are transferred to theholding portion C1. FIG. 5B illustrates the potential states ofrespective portions at the time t1.

At the time t2, the control signals PTX1 on all the rows are controlledfrom the Hi-level to the Lo-level by the vertical scanning circuit 20,and the transfer transistors M1 of the pixels 12 on all the rows areturned off. Thereby, the potential barrier with respect to electrons ofthe transfer transistor M1 part increases, and the transfer operationfrom the photoelectric converter PD to the holding portion C1 ends. FIG.5C illustrates the potential states of respective portions at the timet2. The transfer operations to the holding portions C1 are performed atthe same time in all the pixels, which defines the end of an exposureperiod of the k-th frame.

At the time t3, the control signals PTX3 on all the rows are controlledfrom the Lo-level to the Hi-level by the vertical scanning circuit 20,and the transfer transistors M6 of the pixels 12 on all the rows areturned on. Thereby, the potential barrier with respect to electrons ofthe transfer transistor M6 part decreases, and signal charges which haveremained in the photoelectric converter PD after the transfer operationor have been accumulated in the photoelectric converter PD on and afterthe time t2 are drained to the overflow drain OFD. FIG. 5D illustratesthe potential states of respective portions at the time t3. The chargedrain operations to the overflow drain OFD are performed at the sametime in all the pixels, which defines the start of the exposure periodof the (k+1)-th frame.

At the time t4, the control signals PTX3 on all the rows are controlledfrom the Hi-level to the Lo-level by the vertical scanning circuit 20,and the transfer transistors M6 of the pixels 12 on all the rows areturned off. Thereby, the potential barrier with respect to electrons ofthe transfer transistor M6 part increases, and the charge drainoperation from the photoelectric converter PD to the overflow drain OFDends.

In such a way, drain operations of charges to the overflow drains OFDand transfer operations of signal charges to the holding portions C1 areperformed at the same time in all the pixels to realize the globalelectronic shutter function.

Signal charges of the k-th frame transferred to the holding portion C1of respective pixels 12 are sequentially read out on a row-by-row basison and after the time t4.

At the time t5, the control signals PRES(n) are controlled from theHi-level to the Lo-level by the vertical scanning circuit 20, the resettransistors M3 of the pixels 12 on the n-th row are turned off, and thereset of the FD portions is released.

Also, at the time t5, the control signals PSEL(n) are controlled fromthe Lo-level to the Hi-level by the vertical scanning circuit 20, theselect transistors M5 of the pixels 12 on the n-th row are turned on,and the pixels 12 on the n-th row are selected. Thereby, the signalsVOUT in accordance with the reset electrical potential of the FDportions of the pixels 12 on the n-th row are output to the verticaloutput lines 16.

A control signal PTN is controlled to the Hi-level in the subsequentperiod from the time t6 to the time t7, and thereby reset signals of thepixels 12 on the n-th rows output to the vertical output lines 16 onrespective columns are held in sample-hold capacitors used for resetsignals included in the readout circuit 30, respectively. The controlsignal PTN is a control signal for a switch that controls connection anddisconnection of the sample-hold capacitor used for the N-signal.

At the time t8, the control signals PTX2(n) is controlled from theLo-level to the Hi-level by the vertical scanning circuit 20, and thetransfer transistors M2 of the pixels 12 on the n-th row are turned on.Thereby, the potential barrier with respect to electrons of the transfertransistor M2 part decreases, and signal charges held in the holdingportion C1 are transferred to the holding portion C2. FIG. 5Eillustrates the potential states of respective portions at the time t8.

At the time t9, the control signals PTX2(n) are controlled from theHi-level to the Lo-level by the vertical scanning circuit 20, and thetransfer transistors M2 of the pixels 12 on the n-th row are turned off.Thereby, the potential barrier with respect to electrons of the transfertransistor M2 part increases, and the transfer operation from theholding portion C1 to the holding portion C2 ends. FIG. 5F illustratesthe potential states of respective portions at the time t9.

Signal charges are transferred to the holding portion C2 of the pixels12 on the n-th row, thereby the electrical potential of the FD portionof the pixels 12 on the n-th row becomes a level in which the electricalpotential in accordance with the amount of transferred signal charges isadded to the reset electrical potential, and the signal VOUT inaccordance with the electrical potential is output to the verticaloutput line 16.

A control signal PTS is controlled to the Hi-level in the subsequentperiod from the time t10 to the time t11, and thereby optical signals ofthe pixels 12 on the n-th rows output to the vertical output lines 16 onrespective columns are held in sample-hold capacitors used for opticalsignals included in the readout circuit 30, respectively. The controlsignal PTS is a control signal of a switch that controls connection anddisconnection of the sample-hold capacitor used for the S-signal.

The optical signal and the reset signal held in the sample-holdcapacitor on each column are transferred to the output circuit 50 on acolumn basis according to the control signal from the horizontalscanning circuit 40. In the output circuit 50, the reset signal issubtracted from the optical signal, and a signal from which a noisecomponent has been removed is output as a pixel signal.

At the time t12, the control signals PSEL(n) are controlled from theHi-level to the Lo-level by the vertical scanning circuit 20, and theselect transistors M5 of the pixels 12 on the n-th row are turned off.Thereby, the selection of the pixels 12 on the n-th row is released.Further, similarly at the time t12, the control signals PRES(n) arecontrolled from the Lo-level to the Hi-level by the vertical scanningcircuit 20, and the reset transistors M3 of the pixels 12 on the n-throw are turned on. Thereby, the electrical potentials of the FD portionsof the pixels 12 on the n-th row are reset.

A series of readout operations over the period from the time t5 to thetime t12 are performed on a pixel row basis. In the example of FIG. 4,following to the readout operation of the pixels 12 on the n-th row, areadout operation of the pixels 12 on the (n+1)-th row and a readoutoperation of the pixels 12 on the (n+2)-th row are sequentiallyperformed.

After the completion of the readout operations of the pixels 12 on allthe rows, in a period from the time t13 to the time t14, control signalsPTX1 on all the rows are controlled from the Lo-level to the Hi-level bythe vertical scanning circuit 20 in a similar manner to the period fromthe time t1 to the time t2. Thereby, signal charges accumulated in thephotoelectric converter PD in the period from the time t4 to the time t14 are transferred to the holding portion C1. The time t14 defines theend of the exposure period of the (k+1)-th frame.

Here, the period in which readout operations of respective rows areperformed is defined as “readout period.” FIG. 4 illustrates a readoutperiod of the k-th frame and a readout period of the (k+1)-th frame. Onthe other hand, the period from the time t4 when draining of charges tothe overflow drain OFD is completed to the time t13 when transfer ofsignal charges to the holding portion C1 is started is a period in whichthe photoelectric converter PD accumulates signal charges in the subjectframe. That is, in FIG. 4, a period from the time t4 to the time t13 isthe exposure period of the (k+1)-th frame. Therefore, the readout periodof the k-th frame occurs in a period overlapping with the exposureperiod of the (k+1)-th frame.

In the following description, a timing diagram as illustrated in FIG. 6may be used instead of the timing diagram as illustrated in FIG. 4, ifnecessary. While the timing diagram of FIG. 6 illustrates driving at thesame timing as that in the timing diagram of FIG. 4, other controlsignals than the control signals PTX1 and PTX3 are represented by singlediagonal dotted lines as “readout signal.” This dotted line visuallyrepresents that operations on respective rows are sequentiallyperformed. Since the control signals PTX1 and PTX3 are driven at thesame time on all the rows, each of the control signals PTX1 and PTX3 isrepresented by a single signal in FIG. 6.

Next, a method of driving a photoelectric conversion device according toa first comparative example will be described by using FIG. 7 and FIG.9A. FIG. 7 is a timing diagram illustrating the method of driving thephotoelectric conversion device according to the first comparativeexample. FIG. 9A schematically illustrates potential states ofrespective portions of the pixels 12 in the drive method of the firstcomparative example.

In the drive method of the first comparative example, while the basicoperation timing is the same as the drive method illustrated in FIG. 6,the signal level when the control signal PTX3 is off is set to not theLo-level but an intermediate level (hereafter, denoted as “M-level”)between the Lo-level and the Hi-level.

On the lower side of FIG. 7, a light amount of a light entering aparticular pixel 12 and an amount of charges (electrons) accumulated inthe photoelectric converter PD of the pixel 12 are schematicallyillustrated. In FIG. 7, such a case is assumed that the light amount ofan incident light sharply increases from zero between the time t2 andthe time t3 and then a light whose light amount is sufficient tosaturate the photoelectric converter PD is continuously entering it.

At the time t4, once the control signal PTX3 is controlled from theHi-level to the M-level by the vertical scanning circuit 20 and drainingof charges to the overflow drain OFD ends, accumulation of charges(electrons) in the photoelectric converter PD, that is, the exposureperiod of the (k+1)-th frame starts. Since the light amount of anincident light to the photoelectric converter PD is significantly large,the amount of charges accumulated in the photoelectric converter PDimmediately reaches the saturation charge amount and then becomes aconstant amount.

At the time t13, once the control signal PTX1 is controlled from theLo-level to the Hi-level by the vertical scanning circuit 20 and chargesin the photoelectric converter PD are transferred to the holding portionC1, the photoelectric converter PD becomes empty. Then, in response tothe end of the drain period of charges to the overflow drain OFD similarto the period from the time t3 to the time t4, the exposure period ofthe (k+2)-th frame starts.

FIG. 9A illustrates a potential state when the amount of chargesaccumulated in the photoelectric converter PD reaches the saturationcharge amount. Since the signal level when the transfer transistor M6 isturned off is the M-level, the potential barrier of the transfertransistor M6 is lower than the potential barrier of the transfertransistor M1. Thus, the saturation charge amount of the photoelectricconverter PD is defined by the height of the potential barrier of thetransfer transistor M6. After the photoelectric converter PD issaturated, overflowed electrons are drained to the overflow drain OFDside having a lower potential barrier.

Accordingly, the drive method of the first comparative example has aproblem of reduction in the saturation charge amount of thephotoelectric converter PD.

Next, a method of driving a photoelectric conversion device according toa second comparative example will be described by using FIG. 8 and FIG.9B. FIG. 8 is a timing diagram illustrating the method of driving thephotoelectric conversion device according to the second comparativeexample. FIG. 9B schematically illustrates potential states ofrespective portions of the pixels 12 in the drive method of the secondcomparative example.

In the drive method of the second comparative example, while the basicoperation timing is the same as the drive method of the firstcomparative example illustrated in FIG. 7, the signal level when thecontrol signal PTX3 is off is set to not the M-level but the Lo-level.With respect to the light amount of an incident light to thephotoelectric converter PD, the same situation as in the firstcomparative example illustrated in FIG. 7 is assumed.

In the drive method of the second comparative example, since the signallevel when the control signal PTX3 is off is the Lo-level and thepotential barrier of the transfer transistor M6 is higher than that inthe case of the first comparative example, the saturation charge amountof the photoelectric converter PD is larger than that in the case of thefirst comparative example.

FIG. 9B illustrates a potential state when the amount of chargesaccumulated in the photoelectric converter PD reaches the saturationcharge amount. In the second comparative example, the potential barrierof the transfer transistor M6 is higher than the potential barrier ofthe transfer transistor M1, and the saturation charge amount of thephotoelectric converter PD is defined by the height of the potentialbarrier of the transfer transistor M1. After the photoelectric converterPD is saturated, overflowed electrons leak into the holding portion C1side having a lower potential barrier.

As described above, the readout period of the k-th frame occurs in aperiod overlapping with the exposure period of the (k+1)-th frame. Thus,once leakage of electrons into the holding portion C1 from thephotoelectric converter PD occurs in the exposure period of the (k+1)-thframe, a false signal overlaps with a signal of the k-th frame. In FIG.8, a period in which a false signal is generated is denoted as “falsesignal generation period.”

The exposure period of the k-th frame is a period before the time t1. Inthe example of FIG. 8, since the light amount of an incident light iszero before the time t1, the original data of the k-th frame is zero.However, when electrons leak into the holding portion C1 from thephotoelectric converter PD in the exposure period of the (k+1)-th frame,the leaking electrons are read out as a signal of the k-th frame, andthis results in a false signal.

Accordingly, the drive method of the second comparative example has aproblem of a false signal superimposing on a signal of the previousframe.

Thus, in the present embodiment, in order to overcome the above problemsof the first comparative example and the second comparative example, aphotoelectric conversion device is driven in an operation mode thatperforms global electronic shutter in accordance with a timing diagramillustrated in FIG. 10. FIG. 10 is the timing diagram illustrating amethod of driving the photoelectric conversion device according to thepresent embodiment. The drive method of the present embodiment is thesame as the drive methods of the first comparative example and thesecond comparative example except the operation of the control signalPTX3.

That is, in the present embodiment, at the time t4, the control signalPTX3 is controlled from the Hi-level to a first level that is lower thanthe Hi-level by the vertical scanning circuit 20 to terminate the periodof draining charges to the overflow drain OFD. Then, the control signalPTX3 is continuously changed from a first signal value to a secondsignal value that is lower than the first signal value by the verticalscanning circuit 20 in the exposure period. Thereby, with respect tosignal charges (electrons) held in the photoelectric converter PD, thelevel of the potential barrier formed by the transfer transistor M6changes from the first level to a second level that is higher than thefirst level. In this example, the first level is a level at which thepotential of the transfer transistor M6 with respect to charges(electrons) held in the photoelectric converter PD becomes higher thanthe potential of the photoelectric converter PD. In other words, withthe control signal PTX3 at the first level being supplied, the transfertransistor M6 forms a potential barrier with respect to signal charges(electrons) held in the photoelectric converter PD. Further, the secondlevel is preferably a level at which the potential barrier of thetransfer transistor M6 with respect to charges (electrons) held in thephotoelectric converter PD becomes higher than the first level and lowerthan or equal to the level of the potential barrier of the transfertransistor M1.

That is, with an exposure period elapsing, the potential barrier of thetransfer transistor M6 with respect to electrons gradually increases. Inresponse thereto, the saturation charge amount of the photoelectricconverter PD gradually increases, and the amount of electronsaccumulated in the photoelectric converter PD gradually increases. Inthis period, since the potential barrier of the transfer transistor M6is lower than the potential barrier of the transfer transistor M1, noleakage of electrons into the holding portion C1 from the photoelectricconverter PD occurs. The signal level of the control signal PTX3continuously, gradually decreases so that the height of the potentialbarrier of the transfer transistor M6 finally becomes the same degree asthe height of the potential barrier of the transfer transistor M1 at theend time of the exposure period (time t14). This can ensure thesaturation charge amount of the photoelectric converter PD defined bythe height of the potential barrier of the transfer transistor M1.

Therefore, in the drive method of the present embodiment, it is possibleto prevent generation of a false signal due to leakage of electrons intothe holding portion C1 while suppressing reduction in the saturationcharge amount of the photoelectric converter PD.

Note that, while the height of the potential barrier of the transfertransistor M6 is controlled by the signal level of the control signalPTX3, that is, the gate electrical potential of the transfer transistorM6 in the present embodiment, it may be controlled by the electricalpotential of the overflow drain OFD.

As discussed above, according to the present embodiment, it is possibleto prevent occurrence of a false signal due to leakage of electrons intoa holding portion from a photoelectric converter while suppressingreduction in the saturation charge amount of the photoelectric converterPD.

[Second Embodiment]

A method of driving a photoelectric conversion device according to asecond embodiment of the present invention will be described withreference to FIG. 1 to FIG. 10. Components, control signals, or the likeof the photoelectric conversion device illustrated in the firstembodiment are labeled with the same reference, and the descriptionthereof will be omitted or simplified.

In the present embodiment, another method of driving the photoelectricconversion device according to the first embodiment illustrated by usingFIG. 1 to FIG. 3 will be described. The drive method of the presentembodiment is an application example of a case where a gain of anamplifier circuit included in the readout circuit 30 can be selectedfrom a plurality of set values. As an example, such a case is assumedhere that the gain of the amplifier circuit can be switched amongone-fold, two-fold, four-fold, and eight-fold.

It is assumed that, when the gain is one-fold, the range of the outputsignal level corresponding to the saturation charge amount determined bythe height of the potential barrier on the transfer transistor M1 sideof the photoelectric converter PD is equal to the dynamic range of thepost-stage circuit. In this case, when the gain is two-fold, the dynamicrange of the post-stage circuit will correspond to the range of theoutput signal level corresponding to the charge amount of half thesaturation charge amount of the photoelectric converter PD. That is,when the gain is two-fold or greater, charges of only half or less ofthe saturation charge amount of the photoelectric converter PD arehandled as a final signal.

Thus, in a method of driving the photoelectric conversion deviceaccording to the present embodiment, the potential of the transfertransistor M6 part is controlled in accordance with the gain set valueof the amplifier circuit. For example, when the gain set value istwo-fold or greater, since the light amount of an incident light isrelatively small, driving is performed in accordance with the timingdiagram of the first comparative example illustrated in FIG. 7. At thistime, the M-level of the control signal PTX3 is set to a level such thatcharges above the amount corresponding to the dynamic range of thepost-stage circuit can be accumulated and the potential barrier of thetransfer transistor M6 is lower than the potential barrier of thetransfer transistor M1. Accordingly, it is possible to ensure anecessary dynamic range while suppressing occurrence of a false signaldue to leakage of charges into the holding portion C1 from thephotoelectric converter PD.

On the other hand, when the gain set value is one-fold or less, sincethe light amount of an incident light is relatively larger, eitherdriving is performed in accordance with the timing diagram of the secondcomparative example illustrated in FIG. 8 tolerating occurrence of afalse signal, or driving is performed in accordance with the timingdiagram of the first embodiment illustrated in FIG. 10 taking occurrenceof a false signal into consideration.

As discussed above, according to the present embodiment, it is possibleto ensure a necessary dynamic range while suppressing occurrence of afalse signal due to leakage of charges into the holding portion C1 fromthe photoelectric converter PD even when the gain of the amplifiercircuit of the readout circuit 30 is switched in accordance with thelight amount of an incident light.

[Third Embodiment]

A method of driving a photoelectric conversion device according to athird embodiment of the present invention will be described withreference to FIG. 11 and FIG. 12. Components, control signals, or thelike of the photoelectric conversion device illustrated in the first andsecond embodiments are labeled with the same reference, and thedescription thereof will be omitted or simplified. In the presentembodiment, another method of driving the photoelectric conversiondevice according to the first embodiment illustrated by using FIG. 1 toFIG. 3 will be described.

FIG. 11 is a timing diagram illustrating the method of driving thephotoelectric conversion device according to the present embodiment.FIG. 11 illustrates transitions of the same control signals as in FIG.7, FIG. 8, and FIG. 10 and transitions of the incident light amount tothe pixel 12 and the accumulation charge amount (electron amount) of thephotoelectric converter PD. Illustrated here are a pixel in which thecharge amount generated during an exposure period is greater than thesaturation charge amount determined by the height of the potentialbarrier of the transfer transistor M1 (a pixel with an incident lightamount corresponding to the saturation charge amount or greater) and apixel in which the charge amount generated during an exposure period isequal to the saturation charge amount (a pixel with an incident lightamount corresponding to the saturation charge amount).

In the method of driving the photoelectric conversion device accordingto the present embodiment, as illustrated in FIG. 11, at the time t15 inthe readout period, the level of the control signal PTX3 is changedstepwise. With such an operation, the false signal generation period canbe shorter than that in the case of the second comparative example ofFIG. 8, and a false signal can be reduced.

FIG. 12 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a third comparativeexample. FIG. 12 illustrates transitions of the control signals andtransitions of the incident light amount to the pixel 12 and the chargeaccumulation amount (electron amount) of the photoelectric converter PDsimilarly to FIG. 11. Illustrated here are a pixel in which the chargeamount generated during an exposure period is greater than thesaturation charge amount determined by the height of the potentialbarrier of the transfer transistor M1 (a pixel with an incident lightamount corresponding to the saturation charge amount or greater) and apixel in which the charge amount generated during an exposure period isequal to the saturation charge amount (a pixel with an incident lightamount corresponding to the saturation charge amount).

In the drive method of the third comparative example, at the time t16after the time t15, the level of the control signal PTX3 is changedstepwise. With such a driving, the false signal generation period can beshorter than that in the case of the present embodiment illustrated inFIG. 11. In the drive method of the third comparative example, however,there is a disadvantage in the operation of the pixel 12 including thephotoelectric converter PD which does not reach the saturation chargeamount.

In the driving of the present embodiment illustrated in FIG. 11, theamount of charges accumulated in the photoelectric converter PD of thepixel 12 with an incident light amount corresponding to the saturationcharge amount reaches the saturation charge amount. In the driving ofthe third comparative example illustrated in FIG. 12, however, theamount of charges accumulated in the photoelectric converter PD of thepixel 12 with an incident light amount corresponding to the saturationcharge amount does not reach the saturation charge amount. Thisindicates that, by the driving of the third comparative example, noproper output can be obtained in the pixel 12 in which an incident lightamount is below the light amount corresponding to the saturation chargeamount.

FIG. 11 and FIG. 12 depicts dashed diagonal lines over the timingdiagram of the control signal PTX3. The dashed diagonal line indicates alevel of the control signal PTX3 for maintaining the potential barrierby which no draining of charges from the photoelectric converter PD tothe overflow drain OFD occurs in the pixel 12 in which chargescorresponding to the saturation charge amount are accumulated during anexposure period under a constant incident light amount. When there is aperiod in which the level of the control signal PTX3 exceeds thediagonal line, draining of charges from the photoelectric converter PDto the overflow drain OFD occurs, which results in a reduction inaccumulated charges of the photoelectric converter PD.

In the driving of the third comparative example illustrated in FIG. 12,the level of the control signal PTX3 exceeds the diagonal lineimmediately before the time t3. When draining of charges from thephotoelectric converter PD to the overflow drain OFD occurs in theperiod in which the level of the control signal PTX3 exceeds thediagonal line, the amount of charges accumulated in the photoelectricconverter PD of the pixel 12 with an incident light amount correspondingto the saturation charge amount will not reach the saturation chargeamount. It is therefore preferable to properly set the timing of thestepwise change in the level of the control signal PTX3 so that thelevel of the control signal PTX3 does not exceed the diagonal line as inthe driving of the present embodiment illustrated in FIG. 11.

Accordingly, it is preferable to properly set the level of the controlsignal PTX3 in accordance with the light amount of an incident light.Specifically, when there is an incident light corresponding to thesaturation charge amount defined by the potential of the transfertransistor M1 during an exposure period, the control signal PTX3 iscontrolled so that charges accumulated in the photoelectric converter PDdoes not exceed the potential barrier of the transfer transistor M6.

Note that, while the level of the control signal PTX3 is changed once atthe time t15 in the present embodiment, the level of the control signalPTX3 may be changed twice or more in a divided manner.

As discussed above, according to the present embodiment, it is possibleto obtain an appropriate output in the pixel 12 in which an incidentlight amount is below the light amount corresponding to the saturationcharge amount and ensure a sufficient saturation charge amount of thephotoelectric converter PD.

[Fourth Embodiment]

A method of driving a photoelectric conversion device according to afourth embodiment of the present invention will be described withreference to FIG. 13. Components, control signals, or the like of thephotoelectric conversion device illustrated in the first to thirdembodiments are labeled with the same reference, and the descriptionthereof will be omitted or simplified. In the present embodiment,another method of driving the photoelectric conversion device accordingto the first embodiment illustrated by using FIG. 1 to FIG. 3 will bedescribed.

FIG. 13 is a timing diagram illustrating the method of driving thephotoelectric conversion device according to the present embodiment. Thedrive method of the present embodiment illustrates an applicationexample for a case where a readout period is shorter than an exposureperiod as illustrated in FIG. 13.

As illustrated in FIG. 13, it is assumed that the readout period of thek-th frame ends at the time t17 that is around the middle of theexposure period. In such a case, in the present embodiment, at the timet18 that is slightly after the time t17, the level of the control signalPTX3 is changed stepwise.

From the time t17 to the time t13, the accumulation charge amount of thephotoelectric converter PD reaches the saturation charge amount definedby the control signal PTX1, and leakage of charges into the holdingportion C1 from the photoelectric converter PD then occurs. However, areadout period of the k-th frame ends at the time t17, and chargesleaking into the holding portion C1 from the photoelectric converter PDon and after the time t17 does not result in a false signal.

Accordingly, when the readout period is shorter than the exposureperiod, occurrence of a false signal can be suppressed by setting thetiming at which the level of the control signal PTX3 is changed inaccordance with the timing of the end of the readout period.

As discussed above, according to the present embodiment, it is possibleto ensure a sufficient saturation charge amount of the photoelectricconverter PD while suppressing occurrence of a false signal due toleakage of charges to the holding portion C1 from the photoelectricconverter PD.

[Fifth Embodiment]

A method of driving a photoelectric conversion device according to afifth embodiment of the present invention will be described withreference to FIG. 14. Components, control signals, or the like of thephotoelectric conversion device illustrated in the first to fourthembodiments are labeled with the same reference, and the descriptionthereof will be omitted or simplified. In the present embodiment,another method of driving the photoelectric conversion device accordingto the first embodiment illustrated by using FIG. 1 to FIG. 3 will bedescribed.

FIG. 14 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to the present embodiment. Adrive method of the present embodiment is an example in which the changeof the level of the control signal PTX3 after the readout perioddescribed in the fourth embodiment ends is performed sequentially frompixels on a row where a readout period has ended.

At the time t4, the control signals PTX3 on all the rows are controlledfrom the Hi-level to the M-level by the vertical scanning circuit 20,and draining of charges from the photoelectric converter PD to theoverflow drain OFD is completed. The level of the control signals PTX3are maintained at the M-level until the readout periods of respectivepixel rows end.

For example, on the n-th pixel row, the control signal PTX3(n) ismaintained at the M-level until the time t12 when the readout operationis completed, and the control signal PTX3(n) is changed to the Lo-levelat the time t18 immediately after the time t12. Since the readoutoperation on the n-th pixel row is completed at the time t12, chargesleaking into the holding portion C1 from the photoelectric converter PDafter the time t12 does not result in a false signal.

Similarly, on the (n+1)-th pixel row, the control signal PTX3(n+1) ischanged to the Lo-level at the time t19 after the readout operation onthe pixel row is completed. Further, on the (n+2)-th pixel row, thecontrol signal PTX3(n+2) is changed to the Lo-level at the time t20after the readout operation on the pixel row is completed.

As discussed above, according to the present embodiment, it is possibleto ensure a sufficient saturation charge amount of the photoelectricconverter PD while suppressing occurrence of a false signal due toleakage of charges into the holding portion C1 from the photoelectricconverter PD.

[Sixth Embodiment]

A method of driving a photoelectric conversion device according to asixth embodiment of the present invention will be described withreference to FIG. 15 and FIG. 16. Components, control signals, or thelike of the photoelectric conversion device illustrated in the first tofifth embodiments are labeled with the same reference, and thedescription thereof will be omitted or simplified. In the presentembodiment, another method of driving the photoelectric conversiondevice according to the first embodiment illustrated by using FIG. 1 toFIG. 3 will be described.

FIG. 15 is a timing diagram illustrating a method of driving aphotoelectric conversion device according to a fourth comparativeexample. The fourth comparative example is an example of driving inwhich multiple times (twice in this example) of transfer of charges fromthe photoelectric converter PD to the holding portion C1 are performedduring one frame of an exposure period. That is, one frame of anexposure period is divided into a plurality of periods partitioned by atransfer operation of charges from the photoelectric converter PD to theholding portion C1.

In the fourth comparative example, a case where a readout period isshorter than an exposure period is assumed in the same manner as thecase of the fourth embodiment. In this example, the readout period ofthe k-th frame has ended by the time t17 that is before the time 21around the middle of the exposure period of the (k+1)-th frame thatstarts the time t4 and ends the time t14.

In the period from the time t21 to the time t22 after the readout periodof the k-th frame has ended, the control signal PTX1 is controlled fromthe Lo-level to the Hi-level on all the rows by the vertical scanningcircuit 20. Thereby, charges accumulated in the photoelectric converterPD in the period from the time t4 to the time t22 (first exposureperiod) are transferred to the holding portion C1. This transferoperation is referred to as first transfer. In the transfer operationfrom the photoelectric converter PD to the holding portion C1 performedbetween the time t13 to the time t14, charges accumulated in thephotoelectric converter PD in the period from the time t22 to the timet14 (second exposure period) are transferred to the holding portion C1.This transfer operation is referred to as second transfer. In thisexample, the period denoted as “false signal generation period” in FIG.15 and FIG. 16 is a period in which a false signal may occur. Since thesecond exposure period occurs after the end of the readout period, nofalse signal is caused even when the photoelectric converter PD issaturated.

FIG. 16 is a timing diagram illustrating the method of driving thephotoelectric conversion device according to the present embodiment. Thedriving in the present embodiment is the same as the driving in thefourth comparative example illustrated in FIG. 15 except the driving ofthe control signal PTX3.

In the drive method of the present embodiment, as illustrated in FIG.16, the level of the control signal PTX3 is changed stepwise in themiddle of the first exposure period in the same manner as the drivemethod of the third embodiment. With such an operation, the false signalgeneration period can be shorter than in the case of the fourthcomparative example of FIG. 15, and the false signal can be reduced.Further, the number of times of transfer from the photoelectricconverter PD to the holding portion C1 is two, and thereby thesaturation charge amount of the photoelectric converter PD can besubstantially doubled.

As discussed above, according to the present embodiment, it is possibleto ensure a sufficient saturation charge amount of the photoelectricconverter PD while suppressing occurrence of a false signal due toleakage of charges into the holding portion C1 from the photoelectricconverter PD.

[Seventh Embodiment]

A method of driving a photoelectric conversion device according to aseventh embodiment of the present invention will be described withreference to FIG. 17. Components, control signals, or the like of thephotoelectric conversion device illustrated in the first to sixthembodiments are labeled with the same reference, and the descriptionthereof will be omitted or simplified. In the present embodiment,another method of driving the photoelectric conversion device accordingto the first embodiment illustrated by using FIG. 1 to FIG. 3 will bedescribed.

FIG. 17 is a timing diagram illustrating the method of driving thephotoelectric conversion device according to the present embodiment. Thedriving in the present embodiment is the same as the driving in thefourth comparative example illustrated in FIG. 15 except the driving ofthe control signal PTX3.

In the drive method of the present embodiment, as illustrated in FIG.17, the control signal PTX3 is changed stepwise at the time t21 that isa boundary between the first exposure period and the second exposureperiod. In such an operation, while the saturation charge amount of thephotoelectric converter PD in the first exposure period is less thanthat in the case of the fourth comparative example, the false signalgeneration period can be eliminated. The saturation charge amount of thephotoelectric converter PD in the second exposure period is maintainedto the original saturation charge amount in accordance with the level ofthe control signal PTX1.

As discussed above, according to the present embodiment, it is possibleto ensure a certain saturation charge amount of the photoelectricconverter PD while suppressing occurrence of a false signal due toleakage of charges into the holding portion C1 from the photoelectricconverter PD.

[Eighth Embodiment]

A method of driving a photoelectric conversion device according to aneighth embodiment of the present invention will be described withreference to FIG. 18 and FIG. 19. Components, control signals, or thelike of the photoelectric conversion device illustrated in the first toseventh embodiments are labeled with the same reference, and thedescription thereof will be omitted or simplified. In the presentembodiment, another method of driving the photoelectric conversiondevice according to the first embodiment illustrated by using FIG. 1 toFIG. 3 will be described.

In the method of driving the photoelectric conversion device accordingto the fourth embodiment, when a readout period is shorter than anexposure period, occurrence of a false signal is suppressed by changingthe control signal PTX3 from the M-level to the Lo-level after the endof the readout period. In the drive method of the fourth embodiment,however, another problem may occur when an object changes from a brightstate to a dark state.

FIG. 18 is a timing diagram illustrating the problem of the method ofdriving the photoelectric conversion device according to the fourthembodiment. FIG. 18 illustrates a case where a state of a large lightamount of an incident light changes to a state of a small light amountin the readout period of the k-th frame in the driving in the fourthembodiment.

In FIG. 18, the charge amount accumulated in the photoelectric converterPD is supposed to reach the saturation charge amount determined by thecontrol signal PTX1. During a readout period, however, because of theM-level of the control signal PTX3, the charge amount accumulated in thephotoelectric converter PD remains at a lower charge amount than thesaturation charge amount defined by the control signal PTX3. After theend of a readout period, while the control signal PTX3 is set to theLo-level, the incident light amount is already small at this time and nofurther charge accumulation is performed. As a result, the charge amountfinally accumulated in the photoelectric converter PD does not reach thesaturation charge amount determined by the control signal PTX1.

Thus, in the method of driving the photoelectric conversion deviceaccording to the present embodiment, pixels used for image capture andpixels used for correction are prepared as a plurality of pixels 12forming the imaging region 10. Then, the pixels 12 used for correctionare driven in accordance with the timing diagram illustrated in FIG. 19.It is desirable for the pixels 12 used for correction to be distributedwithin the imaging region 10.

FIG. 19 is a timing diagram illustrating the method of driving thephotoelectric conversion device according to the present embodiment. Inthe driving in the present embodiment illustrated in FIG. 19, thecontrol signal PTX3 is set to the Lo-level during an exposure period forthe pixel 12 used for correction. That is, the potential barrier of thetransfer transistor M6 part is controlled to a level above the potentialbarrier of the transfer transistor M1 part. Thereby, in the pixel 12used for correction, the output level which is supposed to be can beobtained even when a state of a large light amount of an incident lightis changed to a state of a small light amount during a readout period.

The output of the pixel 12 used for correction is used to correct theoutput of a pixel from which the output level which is supposed to behas not been obtained. For example, such a scheme or the like can beapplied that multiplies the parameter, which depends on a shading shapeof the output of the pixel 12 used for correction, by the output of thepixel 12 other than that for correction. Note that, for example,correction of a pixel signal can be performed in a signal processingunit 208 in an imaging system according to a twelfth embodimentdescribed later or in an image processing unit 312 in an imaging systemaccording to a thirteenth embodiment described later.

As discussed above, according to the present embodiment, it is possibleto correct image quality deterioration when an incident light amountchanges from a bright state to a dark state in addition to ensuring asaturation charge amount of the photoelectric converter PD whilesuppressing occurrence of a false signal due to leakage of charges intothe holding portion C1 from the photoelectric converter PD.

[Ninth Embodiment]

A method of driving a photoelectric conversion device according to aninth embodiment of the present invention will be described by usingFIG. 20. Components, control signals, or the like of the photoelectricconversion device illustrated in the first to eighth embodiments arelabeled with the same reference, and the description thereof will beomitted or simplified. In the present embodiment, another method ofdriving the photoelectric conversion device according to the firstembodiment illustrated by using FIG. 1 to FIG. 3 will be described.

In the present embodiment, in a similar manner to the eighth embodiment,a countermeasure to the problem that may occur when an object changesfrom a bright state to a dark state will be described. In the method ofdriving a photoelectric conversion device according to the presentembodiment, the pixels 12 used for correction are driven in accordancewith the timing diagram illustrated in FIG. 20.

FIG. 20 is a timing diagram illustrating the method of driving thephotoelectric conversion device according to the present embodiment. Inthe driving in the present embodiment, as illustrated in FIG. 20, thetiming of transfer of charges from the photoelectric converter PD to theholding portion C1 is different between the pixels 12 used forcorrection and other pixels 12. That is, in the pixel 12 used forcorrection, the control signal PTX1 is controlled to the Hi-level in theperiod from the time t23 to the time t24 in the middle of an exposureperiod of other pixels 12 to transfer charges from the photoelectricconverter PD to the holding portion C1. That is, an exposure period usedfor a correction signal applied to the pixel 12 used for correction iscontrolled in a shorter time than an exposure period of other pixels 12.With such an operation, signals based on charges accumulated during aformer period of the exposure period of other pixels 12 are output fromthe pixels 12 used for correction.

The output of the pixels 12 used for correction is used to correct theoutput of other pixels 12 from which the output level which is supposedto be has not been obtained. For example, such a scheme can be appliedthat, when the output of the pixel 12 used for correction is above acertain amount, replaces outputs of other pixels 12 than that used forcorrection arranged in the periphery thereof with a level in accordancewith the saturation charge amount of the photoelectric converter PD.Note that, for example, correction of a pixel signal can be performed inthe signal processing unit 208 in the imaging system according to thetwelfth embodiment described later or in the image processing unit 312in the imaging system according to the thirteenth embodiment describedlater.

As discussed above, according to the present embodiment, it is possibleto correct image quality deterioration when an incident light amountchanges from a bright state to a dark state in addition to ensuring asaturation charge amount of the photoelectric converter PD whilesuppressing occurrence of a false signal due to leakage of charges intothe holding portion C1 from the photoelectric converter PD.

[Tenth Embodiment]

A method of driving a photoelectric conversion device according to atenth embodiment of the present invention will be described by usingFIG. 21A and FIG. 21B. Components, control signals, or the like of thephotoelectric conversion device illustrated in the first to ninthembodiments are labeled with the same reference, and the descriptionthereof will be omitted or simplified. In the present embodiment,another method of driving the photoelectric conversion device accordingto the first embodiment illustrated by using FIG. 1 to FIG. 3 will bedescribed.

In the present embodiment, in a similar manner to the eighth and ninthembodiments, a countermeasure to the problem that may occur when anobject changes from a bright state to a dark state will be described. Inthe method of driving the photoelectric conversion device according tothe present embodiment, image quality deterioration is detected based onan output image, and the correction thereof is performed.

FIG. 21A is a diagram illustrating an example of an output image of theL-th frame, and FIG. 21B is a diagram illustrating an example of anoutput image of the (L+1)-th frame. In the L-th frame, a light emittingobject such as a lamp emits a light, and as illustrated in FIG. 21A, theoutput of a region depicted with a circle therein is at the saturationlevel of the photoelectric converter PD. In the (L+1)-th frame, whilethe lamp is turned off, as illustrated in FIG. 21B, the output of theregion depicted with a circle therein is at a constant intermediatelevel. The output in the (L+1)-th frame being at the intermediate levelrepresents that a state of a large light amount of an incident lightchanges to a state of a small light amount during the readout period ofthe L-th frame.

In the present embodiment, when a certain size of a region of an outputimage is at a saturation level in the L-th frame and at an intermediatelevel in the next (L+1)-th frame, the output of the region of the(L+1)-th frame is corrected to a saturation level. In such a way, imagequality deterioration occurring when an object changes from a brightstate to a dark state can be reduced. Note that, for example, correctionof a pixel signal can be performed in the signal processing unit 208 inthe imaging system according to the twelfth embodiment described lateror in the image processing unit 312 in the imaging system according tothe thirteenth embodiment described later.

As discussed above, according to the present embodiment, it is possibleto correct image quality deterioration when an incident light amountchanges from a bright state to a dark state in addition to ensuring asaturation charge amount of the photoelectric converter PD whilesuppressing occurrence of a false signal due to leakage of charges intothe holding portion C1 from the photoelectric converter PD.

[Eleventh Embodiment]

A photoelectric conversion device and a method of driving the sameaccording to an eleventh embodiment of the present invention will bedescribed by using FIG. 22. Components, control signals, or the like ofthe photoelectric conversion device illustrated in the first to tenthembodiments are labeled with the same reference, and the descriptionthereof will be omitted or simplified.

In the present embodiment, in a similar manner to the eighth to tenthembodiments, a countermeasure to the problem that may occur when anobject changes from a bright state to a dark state will be described.

In the photoelectric conversion device according to the presentembodiment, each of the pixels 12 forming the imaging region 10 isformed of the circuit illustrated in FIG. 22. FIG. 22 is a circuitdiagram illustrating a configuration example of the pixel 12 of thephotoelectric conversion device according to the present embodiment.

As illustrated in FIG. 22, the pixel 12 of the photoelectric conversiondevice further has a transfer transistor M7 and a holding portion C3 inaddition to the photoelectric converter PD, the transfer transistors M1,M2, and M6, the reset transistor M3, the amplifier transistor M4, theselect transistor M5, and the holding portions C1 and C2. The drain ofthe transfer transistor M7 is connected to the FD node. The node of thesource of the transfer transistor M7 includes a capacitance componentand has a function as a holding portion of charges. FIG. 22 representsthis capacitance as a capacitor element (C3) whose one terminal isconnected to the node. In the following description, this capacitorelement may be denoted as the holding portion C3. The other terminal ofthe capacitor element forming the holding portion C3 is grounded.

In the pixel 12 illustrated in FIG. 22, such potential design that apart of charges overflowed from the transfer transistor M6 side leaksinto the holding portion C3. That is, the holding portion C3 collectsand holds at least a part of charges overflowed from the photoelectricconverter PD. The signal based on the held charges in the holdingportion C3 can be read out by being transferred to the holding portionC2 via the transfer transistor M7.

Also in the photoelectric conversion device according to the presentembodiment, basically, such driving is performed that ensures asaturation charge amount of the photoelectric converter PD whilesuppressing occurrence of a false signal due to leakage of charges intothe holding portion C1 from the photoelectric converter PD asillustrated in FIG. 13 or FIG. 18, for example. In the example ofdriving illustrated in FIG. 18, charges leak into the transfertransistor M6 side from the photoelectric converter PD during a readoutperiod of the k-th frame. A part of such leaking charges is held in theholding portion C3.

In the present embodiment, charges held in the holding portion C3 areutilized as signal charges to overcome the above-described problemoccurring when an object changes from a bright state to a dark state.For example, such a scheme or the like can be applied that adds a signalbased on charges held in the holding portion C1 and a signal based on asignal held in the holding portion C3 with weighting if necessary whenan object changes from a bright state to a dark state. Note that, forexample, correction of a pixel signal can be performed in the signalprocessing unit 208 in the imaging system according to the twelfthembodiment described later or in the image processing unit 312 in theimaging system according to the thirteenth embodiment described later.

As discussed above, according to the present embodiment, it is possibleto correct image quality deterioration when an incident light amountchanges from a bright state to a dark state in addition to ensuring asaturation charge amount of the photoelectric converter PD whilesuppressing occurrence of a false signal due to leakage of charges intothe holding portion C1 from the photoelectric converter PD.

[Twelfth Embodiment]

An imaging system according to the twelfth embodiment of the presentinvention will be described with reference to FIG. 23. Components of thephotoelectric conversion device illustrated in the first to eleventhembodiments are labeled with the same reference, and the descriptionthereof will be omitted or simplified. FIG. 23 is a block diagramillustrating a general configuration of the imaging system according tothe present embodiment.

The photoelectric conversion devices 100 described in the above first toeleventh embodiments can be applied to various imaging systems. Examplesof the applicable imaging systems may include a digital still camera, adigital camcorder, a surveillance camera, a copier machine, a faxmachine, a mobile phone, an on-vehicle camera, an observation satellite,and the like. Further, a camera module having an optical system, such asa lens, and an imaging device may be included in the imaging system.FIG. 23 illustrates a block diagram of a digital still camera as anexample of the above.

The imaging system 200 illustrated as an example in FIG. 23 includes theimaging device 201, a lens 202 that captures an optical image of anobject onto the imaging device 201, an aperture 204 for changing a lightamount passing through the lens 202, and a barrier 206 for protectingthe lens 202. The lens 202 and the aperture 204 form an optical systemthat converges a light onto the imaging device 201. The imaging device201 is any of the photoelectric conversion device 100 described in thefirst to eleventh embodiments and converts an optical image captured bythe lens 202 into image data.

The imaging system 200 further includes a signal processing unit 208that processes an output signal output from the imaging device 201. Thesignal processing unit 208 performs AD conversion that converts ananalog signal output from the imaging device 201 into a digital signal.Further, the signal processing unit 208 performs other operations ofperforming various correction or compression if necessary and outputtingimage data. An AD conversion unit that is a part of the signalprocessing unit 208 may be formed on the semiconductor substrate inwhich the imaging device 201 is provided, or may be formed on adifferent semiconductor substrate from the imaging device 201. Further,the imaging device 100 and the signal processing unit 208 may be formedon the same semiconductor substrate.

The imaging system 200 further includes a memory unit 210 fortemporarily storing image data therein and an external interface unit(external I/F unit) 212 for communicating with an external computer orthe like. The imaging system 200 further includes a storage medium 214such as a semiconductor memory for performing storage or readout ofimaging data and a storage medium control interface unit (storage mediumcontrol I/F unit) 216 for performing storage or readout on the storagemedium 214. Note that the storage medium 214 may be embedded in theimaging system 200 or may be removable.

The imaging system 200 further includes a general control/operation unit218 that controls various computation and controls the entire digitalstill camera and a timing generation unit 220 that outputs varioustiming signals to the imaging device 201 and the signal processing unit208. Here, the timing signal or the like may be input from the outside,and the imaging system 200 may include at least the imaging device 201and the signal processing unit 208 that processes an output signaloutput from the imaging device 201.

The imaging device 201 outputs an imaging signal to the signalprocessing unit 208. The signal processing unit 208 performspredetermined signal processing on an imaging signal output from theimaging device 201 and outputs image data. The signal processing unit208 uses an imaging signal to generate an image.

Application of the photoelectric conversion device 100 of any of thefirst to eleventh embodiments can realize an imaging system that canacquire a good quality image with less noise at a high sensitivity.

[Thirteenth Embodiment]

An imaging system and a movable object according to a thirteenthembodiment of the present invention will be described by using FIG. 24Aand FIG. 24B. FIG. 24A is a diagram illustrating a configuration of theimaging system according to the present embodiment. FIG. 24B is adiagram illustrating a configuration of the movable object according tothe present embodiment.

FIG. 24A illustrates an example of an imaging system related to anon-vehicle camera. The imaging system 300 includes an imaging device310. The imaging device 310 is any of the photoelectric conversiondevices 100 described in the above first to eleventh embodiments. Theimaging system 300 includes an image processing unit 312 that performsimage processing on a plurality of image data acquired by the imagingdevice 310 and a parallax calculation unit 314 that calculates aparallax (a phase difference of parallax images) from the plurality ofimage data acquired by the imaging system 300. Further, the imagingsystem 300 includes a distance measurement unit 316 that calculates adistance to the object based on the calculated parallax and a collisiondetermination unit 318 that determines whether or not there is acollision possibility based on the calculated distance. Here, theparallax calculation unit 314 and the distance measurement unit 316 arean example of a distance information acquisition unit that acquiresdistance information on the distance to the object. That is, thedistance information is information on a parallax, a defocus amount, adistance to an object, or the like. The collision determination unit 318may use any of the distance information to determine the collisionpossibility. The distance information acquisition unit may beimplemented by dedicatedly designed hardware or may be implemented by asoftware module. Further, the distance information acquisition unit maybe implemented by a Field Programmable Gate Array (FPGA), an ApplicationSpecific Integrated Circuit (ASIC), or the like, or may be implementedby combination thereof.

The imaging system 300 is connected to the vehicle informationacquisition device 320 and can acquire vehicle information such as avehicle speed, a yaw rate, a steering angle, or the like. Further, theimaging system 300 is connected with a control ECU 330, which is acontrol device that outputs a control signal for causing a vehicle togenerate braking force based on a determination result by the collisiondetermination unit 318. Further, the imaging system 300 is connectedwith an alert device 340 that issues an alert to the driver based on adetermination result by the collision determination unit 318. Forexample, when the collision probability is high as the determinationresult of the collision determination unit 318, the control ECU 330performs vehicle control to avoid a collision or reduce damage byapplying a brake, pushing back an accelerator, suppressing engine power,or the like. The alert device 340 alerts a user by sounding an alertsuch as a sound, displaying alert information on a display of a carnavigation system or the like, providing vibration to a seat belt or asteering wheel, or the like.

In the present embodiment, an area around a vehicle, for example, afront area or a rear area is captured by using the imaging system 300.FIG. 24B illustrates the imaging system in a case of capturing a frontarea of a vehicle (a capturing area 350). The vehicle informationacquisition device 320 transmits instructions to the imaging system 300or the imaging device 310. Such a configuration can further improve theranging accuracy.

Although the example of control for avoiding a collision to anothervehicle has been illustrated in the above description, the embodiment isapplicable to automatic driving control for following another vehicle,automatic driving control for not going out of a traffic lane, or thelike. Furthermore, the imaging system is not limited to a vehicle suchas the subject vehicle, and can be applied to a movable object (movingapparatus) such as a ship, an airplane, or an industrial robot, forexample. In addition, the imaging system can be widely applied to adevice which utilizes object recognition, such as an intelligenttransportation system (ITS), without being limited to movable objects.

[Modified Embodiments]

The present invention is not limited to the above-described embodiments,and various modifications are possible.

For example, an example in which a part of the configuration of any ofthe embodiments is added to another embodiment or an example in which apart of the configuration of any of the embodiments is replaced with apart of the configuration of another embodiment is one of theembodiments of the present invention.

Further, while photoelectric conversion devices using the photoelectricconverter PD that generates electrons as signal charges have beendescribed as examples in the embodiments described above, the same canapply to a photoelectric conversion device using a photoelectricconverter PD that generates holes as signal charges. In this case, theconductivity type of semiconductor regions forming respective parts ofthe pixel 12 is the opposite conductivity type. Note that the names of asource and a drain of a transistor described in the above embodimentsmay be different in accordance with the conductivity type of thetransistor, a function in interest, or the like, and all or some of thesource and the drain described above may be denoted as the oppositename.

Further, the imaging system illustrated in the above embodiments are anexample of imaging systems to which the photoelectric conversion deviceof the present invention may be applied, the imaging system to which thephotoelectric conversion device of the present invention can be appliedis not limited to the configuration illustrated in FIG. 23 and FIG. 24A.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2017-085494, filed Apr. 24, 2017 which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A photoelectric conversion device comprising: aplurality of pixels each of which includes a photoelectric converterthat generates charges by photoelectric conversion, a first transferunit that is connected to the photoelectric converter and transferscharges in the photoelectric converter to a first holding portion, asecond transfer unit that is connected to the first holding portion andtransfers charges in the first holding portion to a second holdingportion, an amplifier unit that is connected to the second holdingportion and outputs a signal based on charges held in the second holdingportion, and a third transfer unit that is connected to thephotoelectric converter and transfers charges in the photoelectricconverter to a node as an overflow drain to which a power source voltageis supplied; and a control unit that, in an exposure period in whichsignal charges are accumulated in the photoelectric converter, changes apotential barrier formed by the third transfer unit with respect to thesignal charges accumulated in the photoelectric converter from a firstlevel to a second level that is higher for the signal charges than thefirst level.
 2. The photoelectric conversion device according to claim1, wherein the first level is lower than a level of a potential barrierformed by the first transfer unit with respect to the signal chargesaccumulated in the photoelectric converter in the exposure period. 3.The photoelectric conversion device according to claim 1, wherein thesecond level is lower than or equal to a level of a potential barrier ofthe first transfer unit with respect to the signal charges.
 4. Thephotoelectric conversion device according to claim 1, wherein theexposure period includes a first period in which charges held by thefirst holding portion of pixels arranged on a plurality of rows of theplurality of pixels are transferred to the second holding portionsequentially on a row-by-row basis.
 5. The photoelectric conversiondevice according to claim 4, wherein the exposure period furtherincludes a second period in which accumulation of the signal charges iscontinued after the first period.
 6. The photoelectric conversion deviceaccording to claim 5, wherein the control unit changes the potentialbarrier of the third transfer unit with respect to the signal chargesfrom the first level to the second level in the first period of theexposure period.
 7. The photoelectric conversion device according toclaim 6, wherein the control unit changes the potential barrier of thethird transfer unit with respect to the signal charges from the firstlevel to the second level sequentially from a pixel on a row on which anoperation of transferring charges held by the first holding portion tothe second holding portion has ended.
 8. The photoelectric conversiondevice according to claim 5, wherein the control unit changes thepotential barrier of the third transfer unit with respect to the chargesfrom the first level to the second level after the first period ends. 9.The photoelectric conversion device according to claim 5, wherein thefirst transfer unit transfers charges of the photoelectric converter tothe first holding portion between the first period and the secondperiod.
 10. The photoelectric conversion device according to claim 1,wherein the control unit changes the potential barrier of the thirdtransfer unit with respect to the signal charges continuously from thefirst level to the second level.
 11. The photoelectric conversion deviceaccording to claim 1, wherein the control unit changes the potentialbarrier of the third transfer unit with respect to the signal chargesstepwise from the first level to the second level.
 12. The photoelectricconversion device according to claim 1, wherein, at the end of theexposure period, the potential barrier of the third transfer unit withrespect to the signal charges is the same level as the potential barrierof the first transfer unit with respect to the signal charges.
 13. Thephotoelectric conversion device according to claim 1, wherein thecontrol unit controls a level of the potential barrier of the thirdtransfer unit with respect to the signal charges in accordance with alight amount of an incident light.
 14. The photoelectric conversiondevice according to claim 13, wherein the control unit controls apotential of the third transfer unit with respect to the signal chargesso that, when an amount of charges accumulated in the photoelectricconverter during the exposure period under a constant light amount isequal to a saturation charge amount defined by the potential barrier ofthe first transfer unit, charges accumulated in the photoelectricconverter does not exceed the potential barrier of the third transferunit.
 15. The photoelectric conversion device according to claim 1,wherein the plurality of pixels include pixels used for image captureand pixels used for correction, and wherein the control unit controls apotential barrier of the third transfer unit with respect to chargesaccumulated in the photoelectric converter to a third level higher thanor equal to a potential barrier of the first transfer unit during theexposure period for the pixels used for correction.
 16. Thephotoelectric conversion device according to claim 1, wherein theplurality of pixels include pixels used for image capture and pixelsused for correction, and wherein an exposure period for a correctionsignal applied to the pixels used for correction is shorter than theexposure period applied to the pixels used for image capture.
 17. Thephotoelectric conversion device according to claim 1, further comprisingan amplifier circuit that amplifies a signal based on the signal chargesat a gain that can be selected from a plurality of set values, whereinthe control unit sets the first level in accordance with the gain. 18.The photoelectric conversion device according to claim 1, wherein thecontrol unit controls the potential barrier of the third transfer unitwith respect to the signal charges by a voltage supplied to a gate of atransistor forming the third transfer unit.
 19. The photoelectricconversion device according to claim 1, wherein the control unitcontrols the potential barrier of the third transfer unit with respectto the signal charges by a voltage supplied to the drain portion. 20.The photoelectric conversion device according to claim 1, wherein eachof the plurality of pixels further includes a third holding unit thatcollects and holds at least a part of charges overflowed from thephotoelectric converter.
 21. The photoelectric conversion deviceaccording to claim 1, wherein the exposure period includes a periodwhich starts at a first time point when both the first and thirdtransfer units become in an off state on at least one of the first andthird transfer units turning off and ends at a second time point whenthe first transfer unit turns on for a first time after the first timepoint, the control unit changes the potential barrier from the firstlevel to the second level within the period.
 22. A photoelectricconversion device comprising: a plurality of pixels each of whichincludes a photoelectric converter that generates charges byphotoelectric conversion, a first transfer unit that is connected to thephotoelectric converter and transfers charges in the photoelectricconverter to a first holding portion, a second transfer unit that isconnected to the first holding portion and transfers charges in thefirst holding portion to a second holding portion, an amplifier unitthat is connected to the second holding portion and outputs a signalbased on charges held in the second holding portion, and a thirdtransfer unit that is connected to the photoelectric converter andtransfers charges in the photoelectric converter to a node as an overflow drain to which a power source voltage is supplied; an amplifiercircuit that amplifies a signal based on signal charges by a gain thatcan be selected from at least a first gain and a second gain differentfrom the first gain; and a control unit that controls a potentialbarrier formed by the third transfer unit with respect to the signalcharges accumulated in the photoelectric converter to a first level whenthe gain is the first gain and controls a potential barrier of the thirdtransfer unit with respect to the signal charges to a second leveldifferent from the first level when the gain is the second gain.
 23. Thephotoelectric conversion device according to claim 22, wherein the firstgain is higher than the second gain, and wherein the first level islower than the second level.
 24. The photoelectric conversion deviceaccording to claim 22, wherein at least one of the first level and thesecond level is lower than a level of a potential barrier formed by thefirst transfer unit with respect to the signal charges accumulated inthe photoelectric converter in an exposure period.
 25. The photoelectricconversion device according to claim 22, wherein the control unitcontrols the potential barrier of the third transfer unit with respectto the signal charges by a voltage supplied to a gate of a transistorforming the third transfer unit.
 26. The photoelectric conversion deviceaccording to claim 22, wherein the exposure period includes a periodwhich starts at a first time point when both the first and thirdtransfer units become in an off state on at least one of the first andthird transfer units turning off and ends at a second time point whenthe first transfer unit turns on for a first time after the first timepoint, the control unit changes the potential barrier from the firstlevel to the second level within the period.
 27. An imaging systemcomprising: the photoelectric conversion device according to claim 1;and a signal processing unit that processes signals output from theplurality of pixels of the photoelectric conversion device.
 28. Theimaging system according to claim 27, wherein, when an image acquiredfrom the photoelectric conversion device includes a region in which anoutput is at a saturation level in a frame and an output is at anintermediate level in a next frame, the signal processing unit correctsan output of the region of the next frame to a saturation level.
 29. Amovable object comprising: the photoelectric conversion device accordingto claim 1; a distance information acquisition unit adapted to acquiredistance information of a distance to an object based on signals fromthe photoelectric conversion device; and a controlling unit adapted tocontrol the movable object based on the distance information.
 30. Animaging system comprising: the photoelectric conversion device accordingto claim 22; and a signal processing unit that processes signals outputfrom the plurality of pixels of the photoelectric conversion device. 31.The imaging system according to claim 30, wherein, when an imageacquired from the photoelectric conversion device includes a region inwhich an output is at a saturation level in a frame and an output is atan intermediate level in a next frame, the signal processing unitcorrects an output of the region of the next frame to a saturationlevel.
 32. A movable object comprising: the photoelectric conversiondevice according to claim 22; a distance information acquisition unitadapted to acquire distance information of a distance to an object basedon signals from the photoelectric conversion device; and a controllingunit adapted to control the movable object based on the distanceinformation.